1. Field of the Invention
The present invention relates to a process for the mechanical roughening of the surface of a printing plate substrate comprising aluminum or an aluminum alloy by wet brushing. The present invention also relates to a cylinder brush for carrying out the process.
2. Description of Related Art
The production of substrates for photosensitive printing plates requires roughening of the substrate surface, which is generally done by carrying out the roughening first mechanically and then electrochemically in an electrolyte. The mechanical roughening can be effected by brushing with a wire brush, sandblasting, wet and dry brushing techniques, ball milling and other similar techniques.
The printing process is carried out using a printing plate having an essentially even surface. The printing plate is prepared by roughening and chemical treatment. The chemical treatment is either a purely chemical treatment in a solution or an electrochemical treatment. This preparation provides a printing surface which accepts only the oily printing ink and repels water in the printing areas, and conversely accepts water and repels the printing ink in the nonprinting areas.
To carry out the printing process, the processed printing plate is moistened and the printing ink applied. In offset printing, the printing plate is then pressed against a rubber blanket which transfers the printed image inked by the printing ink to the paper to be printed. In the case of direct printing, the printing plate is pressed directly against the paper. High-quality images without any noticeable preferred direction are desirable. An undesired direction may occur when the surface of the printing plate has a nonuniform structure. Furthermore, it is desirable to produce consistent high-quality images when the same printing plate and the same printing plate type are used. A nonreproducible image may occur when the surface of the printing plate is not of constant quality or does not retain its surface quality in a long print run.
A photosensitive printing plate is produced by applying a photosensitive layer to a roughened substrate, which generally comprises aluminum or an aluminum alloy. As aforementioned, the surface is roughened either mechanically, purely chemically or electrochemically or by a combination of two or three of these techniques. The roughening is usually followed by an etch process, anodization and treatment of the surface which renders the surface hydrophilic if required. Finally, a photosensitive layer is applied to the roughened surface in order to obtain a presensitized printing plate. The printing plate is exposed imagewise to actinic radiation, developed and conserved in order to obtain a processed printing plate, which is then mounted in a printing press for printing.
A number of processes, as already mentioned above, are generally known in the area of the roughening of the surface of substrates for printing plates. These processes, however, have disadvantages. For example, in the case of ball milling, the edges of the substrate have a roughness which differs from that of the center of the substrate, so that the plate edges must be cut off in order to ensure uniform roughness over the width of the substrate. Brushing with a wire brush typically gives rise to the problem that the roughness is oriented, the orientation being in the direction of rotation of the wire brush. Characteristic center line average values R.sub.a of wire-brushed material are 0.25 .mu.m in the running direction of a substrate strip and 0.45 .mu.m transverse to the strip running direction. Thus, there is an overall 50% difference between the two directions perpendicular to one another.
In the case of sandblasting substrates, the spray jets are subject to considerable wear due to the abrasive sand particles, and, as a result, must frequently be replaced. In electrochemical roughening, the electricity consumption is high and there are typically problems with the disposal of the acid-containing solutions which contain dissolved aluminum ions. If purely chemical roughening is employed, the residence time of the substrate in the chemical bath is relatively long in comparison with electrochemical roughening.
In the case of wet brushing with a suspension in which small abrasive particles are dispersed in an aqueous solution, the suspension is rubbed against the surface of the substrate by means of a brush or a set of brushes. The bristles of the brushes are usually plastic, such as nylon or polypropylene, and the wet brushing is carried out until the desired surface roughness is reached. The disadvantage with this technique is that the plastic bristles are subject to considerable abrasion and the edges of the substrate cut off the bristles so that the bristles in the edge regions are considerably shorter than those in the other regions of the brush. As a result, if a broader substrate strip is then brushed, the shorter bristles cannot sufficiently roughen the broader substrate strip in these edge regions. This leads to considerably less roughening in the edge regions when compared with the other regions.
Using wire brushes instead of plastic brushes for the wet brushing results in the above-mentioned disadvantages, specifically, the surface of the substrate is not uniform in terms of roughness but has a preferred orientation. The roughness of the substrate surface also varies with the wear of the bristles.
The wet brushing of aluminum substrates is described, for example, in U.S. Pat. No. 3,929,591 and U.S. Pat. No. 4,477,317.
U.S. Pat. No. 4,714,528 discloses a cylinder brush which is used for wet brushing. A substrate comprising aluminum or an aluminum alloy which contains 99% by weight of aluminum and small amounts of silicon, iron, copper, zinc, manganese, magnesium, chromium, lead, bismuth, calcium, indium, gallium, nickel and the like, is processed with a suspension under high liquid pressure which is applied at an angle to the surface of the substrate. The suspension contains finely milled powder of abrasive substances and, if required, may furthermore contain an acid or an alkali. This roughening step by means of a suspension is followed by a brush treatment, the cylinder brush comprising nylon fibers, polypropylene fibers, animal hair, steel wire or the like. The fibers or bristles have a uniform length and are mounted uniformly over the base part of the cylinder brush. The length of the bristles or fibers is from 10 to 150 mm and the diameter of the individual fibers or wires ranges from 0.1 mm to 1.5 mm. The cylinder brush is operated at a speed in the range from 200 to 2000 revolutions per minute. The abrasive suspension is sprayed through a spray nozzle onto the substrate under high liquid pressure before the substrate passes the cylinder brushes. The brushes are pressed against the substrate so that the substrate surface is roughened at a constant pressure between the support rolls and the cylinder brushes. After the roughening, the roughened surface of an aluminum substrate has a center line average value R.sub.a of from 0.3 to about 1.2 .mu.m, in particular from 0.35 to 0.8 .mu.m.